Multimedia applications in a Wi-Fi network can benefit from Quality of Service (QoS) functionality. QoS enables Wi-Fi devices to prioritize traffic and optimizes the way shared network resources are allocated among different applications. Without QoS, all applications running on different devices have an equal opportunity to transmit data frames. That may work well for data traffic from applications such as web browsers, file transfers, or email, but it is less appropriate for multimedia applications. Voice over Internet Protocol (VoIP), video streaming and interactive gaming, for example, are highly sensitive to latency increases and throughput reductions, and can benefit from QoS.
To meet the need for quality of service, the QoS task group of the Wi-Fi Alliance published the Wi-Fi MultiMedia (WMM) specifications 1.1. This priority-based method was adopted by the Wi-Fi Alliance as a baseline set of QoS features to be certified in devices that support new QoS capabilities defined in Institute of Electrical and Electronic Engineers (IEEE) 802.11e. WMM provides QoS via Enhanced Distributed Coordination Function (EDCF), the ability to perform four queues of priority QoS, and optional features such as admission control and burst acknowledgement. EDCF is based on probability and statistically favors high-priority traffic. EDCF's channel-access scheme is Enhanced Distributed Channel Access (EDCA). Two access mechanisms are specified by IEEE 802.11e—EDCA and Hybrid Coordinate Channel Access (HCCA). The Wi-Fi Multimedia—Scheduled Access (WMM-SA) specification supports HCCA-based centralized scheduling in addition to EDCA.
The transmission of frames carrying time-sensitive content (“content frames”) such as audio and video from a station (STA) during a communication session requires the transmission of signaling frames responsible for the creation, maintenance and termination of the communication session. For example, a STA and a teleconferencing server may exchange signaling frames to establish a communication session before a teleconferencing application in the STA can participate in a teleconference managed by the server. The server may also expect to properly exchange signaling frames with the STA in order to terminate the communication session.
WMM specifications version 1.1 prioritizes traffic demands from different applications in order to extend Wi-Fi's high quality end-user experience from data connectivity to voice, music, and video applications. In systems supporting QoS, the transmission of content frames during a communication session may be prioritized over the transmission of other information. For example, if the QoS mechanism of a Wi-Fi system is implemented according to the WMM/IEEE 802.11e—EDCA specification, the signaling frames may be assigned priorities that are at most equal to the priority assigned to the content frames. If, for any reason, content frames are jamming the wireless medium (WM) over which they are transmitted, the timely transmission of both content frames and signaling frames is degraded. The successful transmission of signaling frames to resolve the WM's jamming may be delayed due to the jamming itself, and the result may be a prolonged time during which quality of service for the voice streams over the WM is degraded.
In an Infrastructure network, a WMM-enabled AP and a WMM-enabled STA negotiate WMM QoS parameters on association and re-association. Following an association, the AP may advertise changes to QoS parameters at any time in beacon frames.
A WMM-enabled AP includes either a WMM Information Element (IE) or a WMM Parameter Element (PE) in every beacon it transmits. A WMM IE includes a QoS information field and indicates capability or use of WMM according to context, and is described in section 2.2.1 of WMM specifications 1.1. A WMM PE contains a QoS information field and EDCA parameter records for each of the four access categories and is described in section 2.2.2 of WMM specifications 1.1.
A WMM-enabled STA determines the WMM capability of an AP with which it wishes to associate before transmitting an association request to it. It may do this either passively, by receiving a beacon frame, or actively, by transmitting a probe request to it. The STA then sends an association request to the AP and includes a WMM IE in the association request. On receipt of an association request, the AP responds by transmitting an association response that includes a WMM PE.
WMM leaves the network owner free to choose the most appropriate network-wide policy and to decide which access categories have priority. For instance, a network owner may prefer to give priority to video streaming over voice. A customized policy for the access categories can be set through an interface in which default priority levels for access categories can be modified.
With WMM, applications assign each MAC Service Data Unit (MSDU) to a given access category. MAC headers of frames of type “data” and subtype “QoS data” or “QoS null” carry a QoS control field, which contains 3 bits defining the user priority assigned to the frame. Both content MAC frames and signaling MAC frames are of type “data” and typically are of subtype “QoS data”.
MSDUs are then sorted and added to one of four independent transmit queues (one per access category; i.e., voice, video, best effort, or background) in the STA. The STA has an internal collision resolution mechanism to address collision among different queues, which selects the frames with the highest priority to transmit. The same mechanism deals with external collision, to determine which STA should gain time for transmission.
The time gained by the STA depends on the access category and the Physical layer (PHY) rate. For instance, the limit on the duration of the time ranges from 0.2 ms (background priority) to 3 ms (video priority) in an IEEE 802.11a/g network, and from 1.2 ms to 6 ms in an IEEE 802.11b network. This bursting capability enhances the efficiency for high data rate traffic, such as Audio/Video (AV) streaming. In addition, devices operating at higher PHY rates are not penalized when devices that support only lower PHY rates (e.g. because of distance) contend for access to the medium.
With the default priorities used, voice-related MSDUs have the highest priority to be transmitted and are more likely to be transmitted than MSDUs of other types. However, a wireless medium used for the transmission has a limited capacity and therefore there is a limit to the number of voice-related MSDUs it can handle in any given time.
If the demand to transmit voice-related MSDUs is higher than the capacity of the WM, for example, because the number of voice streams that are concurrently handled by the WM is too high, some percentage of the voice-related MSDUs will be dropped and not transmitted. A network management module may attempt to handle such a situation by terminating one or more of the voice streams.
In order to do so, the network management module may have to initiate transmission of one or more voice-signaling MSDUs to STAs participating in one or more of the voice streams. Due to the saturation of the WM, such voice-signaling MSDUs may also be dropped and the network management module may have to retry transmission until it succeeds. The result may be a prolonged time during which quality of service for the voice streams over the wireless link is degraded.
A similar degradation in quality of service may occur if a WM is saturated with video streams and/or a combination of video and audio streams.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.